Thermoplastic foam and method for production thereof

ABSTRACT

A thermoplastic foam is provided which contains a thermoplastic resin and a layered silicate and in which foam cells and the layered silicate are evenly and finely dispersed. 
     A volume-expansible gas or heat decomposable blowing agent is incorporated into interlayer spaces of a layered silicate in a composite material containing 100 parts by weight of a thermoplastic resin and 0.1-50 parts by weight of the layered silicate, and the gas or blowing agent is allowed to expand in volume or thermally decompose to form foam cells  6 , so that a thermoplastic foam is obtained in which the layered silicate has an average interlayer spacing of over 60 Å when determined by an X-ray diffractometry.

TECHNICAL FIELD

The present invention relates to a thermoplastic foam which contains athermoplastic resin and a layered silicate and has uniformly fine foamcells evenly distributed therein and also to a method for productionthereof.

BACKGROUND ART

Dispersion of a layer silicate in thermoplastic resins is known toimprove their mechanical, thermal, gas barrier or other properties. Inthe layered silicate constituting clay minerals, extremely fine, flakycrystals are held together by ionic bonding. The above-describedproperties of thermoplastic reins can be improved by disintegrating thisaggregate structure by a chemical or physical means to thereby evenlydisperse the flaky crystals throughout the thermoplastic resins.

For example, Japanese Patent Publication No. Hei 8-22946 discloses thata polyamide resin structure containing evenly-dispersed flakes of alayered silicate can be formed by intercalating aminocarboxylic acidinto the layered silicate to initially enlarge spacings between adjacentlayers, then inserting ε-caprolactam, which is a monomeric moiety ofpolyamide, into interlayer spaces and concurrently allowing it toundergo polycondensation.

However, in general, it is extremely difficult to achieve uniformdispersion of the layer silicate in a matrix of a polymer if it isdisimilar to polyamide and its monomer can not be inserted intointerlayer spaces of the layered silicate. Various attempts have beenmade to solve this problem.

For example, Japanese Patent Laying-Open No. Hei 9-183910 discloses amethod for dispersing a layered silicate in a polymer by mixing, in amolten state, an organic dispersion incorporating anorganically-modified layered silicate swelled and dispersed therein witha vinyl polymer compound. Japanese Patent Laying-Open No. Hei 10-182892discloses that melt neading of an organically-modified layered silicate,a polyolefin oligomer containing a hydrogen-linkable functional groupand a polyolefin polymer results in the preparation of a polyolefinicresin composite material in which a spacing between adjacent layers ofthe layered silicate is infinitely swelled in the polymer.

Meanwhile, resins have been conventionally used in the form of foams toreduce the weight or cost of the resins or to provide decorativeappearances thereto. Incorporation of inorganic fillers in such foamshas also been conventional to improve mechanical strength, heatinsulation performance, impact absorption performance of the foams. Forexample, Japanese Patent Laying-Open No. Hei 8-143697 describes thatincorporation of a layered silicate in a polypropylene foam compositionresults in the improved strength or other physical properties of such afoam.

However, the method described in Japanese Patent Laying-Open No. Hei9-183910 requires the use of a solvent. The resulting composite materialexhibits the insufficient strength, such as in flexural modulus,probably due to the insufficient removal of a residual solvent. Also,the inclusion of complex steps, such as of dissolving a polymer,swelling the organically-modified layered silicate and removing thesolvent, makes this prior art impracticable from an industrial point ofview.

Also, the material described in Japanese Patent Laying-Open No. Hei10-182892 as containing the layered silicate in the form of crystallineflakes dispersed evenly in a polymer has been found extremely difficultfor practical use as an industrial material.

That is, because a reaction between a functional group in the polyolefinoligomer and a hydroxyl group on a surface of the layered silicate iscaused to occur during the melt kneading, the hydroxyl group of thelayered silicate is not necessarily treated in an effective manner bythe functional group. Accordingly, a large amount of polyolefin oligomeris required to achieve uniform dispersion of the layered silicate inpractice. The high loading of such an oligomer component in the polymeris undesirable in terms of physical properties and cost.

Japanese Patent Laying-Open No. Hei 8-143697 discloses that apolypropylene foam composition if including a layered silicate with ablowing agent adsorbed therein provides a polypropylene foam with a highexpansion ratio and high strength. However, disintegrating anaggregation structure of the layered silicate and evenly dispersing theresulting flaky crystals in the resin are left out of consideration. Thedisclosed technique thus fails to obtain a sufficient effect of theloaded layered silicate. Also, a specific blowing agent must be heldadsorbed to the layered silicate. The requirement of a multi-stagetreatment thus reduces productivity. Further, the use of a silanecoupling agent is essential. This increases the cost. The unstablenature of the silane coupling agent which is highly linkable to amoisture in the air reduces handleability.

In view of the problems encountered with the above-describedconventional thermoplastic foam compositions each containing athermoplastic resin and a layered silicate and methods for productionthereof, the present invention is directed to provide a thermoplasticfoam which comprises a thermoplastic resin and a layered silicate and inwhich foam cells and layered silicate are evenly and finely dispersed,and also to provide a method for production thereof.

DISCLOSURE OF THE INVENTION

In accordance with a broad aspect of the present invention, athermoplastic foam is provided which is characterized as containing, asmain components, 100 parts by weight of a thermoplastic resin and 0.1-50parts by weight of a layered silicate.

In a particular aspect of the present invention, the layered silicate inthe thermoplastic foam has an average interlayer spacing of over 60 Åwhen determined by X-ray diffractometry.

In another particular aspect of the present invention, X/(Y−1)^(1/3)does not exceed 30 (μm), where X (μm) is an average cell diameter and Yis an expansion ratio.

In a further particular aspect of the present invention, a polyolefinresin is used for the theremoplastic resin.

In a further particular aspect of the present invention, at least oneselected from smectite clay minerals and micas is used for the layeredsilicate.

In a further particular aspect of the present invention, at least oneselected from the group consisting of polyethylene, ethylene-α-olefincopolymer, ethylene-propylene copolymer, polypropylene andpropylene-α-olefin copolymer is used for the polyolefin resin.

In a broad aspect of the method for production of a thermoplastic foamin accordance with the present invention, the method includes the stepsof impregnating a volume-expansible chemical substance into interlayerspaces of a layered silicate in a composite material which contains 100parts by weight of a thermoplastic resin and 0.1-50 parts by weight ofthe layered silicate, and allowing the chemical substance to expand involume within the composite material for formation of cells therein sothat a thermoplastic foam is obtained.

In a particular aspect of the production method, the step ofimpregnating the chemical substance is performed by impregnating, undera high pressure, the chemical substance that assumes a gaseous form atordinary temperature and pressure. Also, the expansion in volume of thechemical substance is effected by vaporizing it within the compositematerial.

In another particular aspect of the production method in accordance withthe present invention, the chemical substance that assumes a gaseousform at ordinary temperature and pressure is in its supercritical stateimpregnated into the composite material.

In a further broad aspect of the production method of the presentinvention, a method for production of a thermoplastic foam is providedwhich is characterized as including the steps of preparing a compositematerial which comprises 100 parts by weight of a thermoplastic resinand 0.1-50 parts by weight of a layered silicate that contains a heatdecomposable blowing agent between its layers, and heating the compositematerial to a temperature sufficiently high to cause decomposition ofthe blowing agent so that a cell structure is formed.

In a further broad aspect of the production method in accordance withthe present invention, the method is provided including the steps ofimpregnating an expansible chemical substance into a thermoplastic resincomposition which contains 100 parts by weight of a thermoplastic resinand 0.1-50 parts by weight of a layered silicate under a high pressurewithin an injection molding machine having a cavity and, subsequent toinjection of the thermoplastic resin composition impregnated with thechemical substance into the cavity of the injection molding machine,allowing the cavity to enlarge.

In a particular aspect of the production method, the chemical substancethat assumes a gaseous form at ordinary temperature and pressure is inits supercritical state impregnated into the thermoplastic resincomposition within the injection molding machine.

Also in the production method, the layered silicate is preferablytreated so as to render its interlayer spaces hydrophobic.

Details of the present invention are given below.

In the present invention, it is most noticeable that a fine cellstructure can be readily formed by allowing the chemical substancepresent in the interlayer spaces of the layered silicate to expand involume within a resin so that flaky crystals of the layered silicate areevenly dispersed in an organic polymer.

The mechanism involved in the present invention is now described indetail.

As schematically shown in FIG. 1 and FIG. 2, flaky crystals 2, 3 of alayered silicate 1, e.g., montmorillonite shown in FIG. 1, are generallyconstituted by tetrahedrons with four oxygen ions arranged to surround acentral silicon or other ion, octahedrons with six oxygen ions arrangedto surround a central aluminum or other ion, and OH groups. A sodium,calcium or other cation arranged on a crystal surface (B) binds suchflaky crystals 2, 3 by ionic bonding.

Since the sodium, calcium or other ion on the crystal surface (B) isgenerally ion-exchangeable with a cationic material, the insertion ofvarious cationic materials into interlayer spaces is enabled. Utilizingthis nature, the above-identified ion can be ion-exchanged with acationic surfactant. The use of a highly nonpolar cationic species forthe cationic surfactant lowers the polarity of the layered silicate (B)to thereby facilitate dispersion of the layered silicate in a nonpolarpolymer.

Also, if such flakes of the layered silicate are to be dispersed, theflaky crystals 2, 3 must be spaced away from each other by applying tointerlayer spaces such an energy that counteracts or lowers anelectrical interaction between the crystal surfaces (B).

To this end, the present invention involves subjecting a compositematerial comprising a thermoplastic resin and a layered silicate to atreatment whereby a volume-expansible chemical substance is insertedinto the interlayer spaces of the layered silicate, or alternatively, aheat decomposable substance is incorporated in the interlayer spaces ofthe layered silicate. The subsequent expansion in volume of the chemicalsubstance or thermal decomposition of the heat decomposable blowingagent provides an energy sufficient to separate the flaky crystals fromeach other.

Also in accordance with the present invention, while the chemicalsubstance that assumes a gaseous form, or a gas released viadecomposition of the heat decomposable blowing agent expands within thecomposite material comprising the thermoplastic resin and the layeredsilicate 5, the flaky crystals 5A act as barriers, as schematicallyshown in FIG. 3. This suppresses excessive diffusion of the gas throughmolecular chains 4 of the thermoplastic resin. As a consequence, a foamis obtained including foam cells 6 finely and evenly dispersed therein.The suppression of excessive diffusion of the gas results in the reducedoccurrence of gas escaping. A higher expansion ratio can be obtained asa consequence.

Conventionally, various measures, e.g., size reduction of a blowingagent or loading of a minute additive serving as a nucleus of expansion,have been taken in the production of foams to suppress the reduction instrength thereof that may result from a marked size increase of foamcells. However, the present invention provides fine and uniform foamcells in a simple manner by a technique that is totally different fromsuch conventional ones.

Preferably, the thermoplastic foam in accordance with the presentinvention has an expansion ratio Y in the range of 1.01-100. Within thisrange of expansion ratio Y, an average cell diameter X (μm) of thethermoplastic foam preferably satisfies the following relationship (1).Average cell diameter X/(expansion ratio Y −1)^(1/3)≦30   (1 )

If a numerical value calculated from a left side of the relationship (1)exceeds 30, the reduction in physical properties of the thermoplasticfoam, such as in insulation performance, compressive strength or bendingcreep, may result.

In the present invention, the layered silicate refers to a silicatemineral which has plural layers comprised of a number of fine flakycrystals and incorporates exchangeable cations between its layers. Theflaky crystal generally has a thickness of about 1 nm and a ratio(hereinafter referred to as an aspect ratio) of a major diameter to thethickness in the approximate range of 20-200. In the layered silicate,these fine flaky crystals are held together by ionic bonding.

The above-described layered silicate incorporating exchangeable cationsbetween its layers is not particularly specified in type. Examples ofsuch layered silicates include smectite clay minerals such asmontmorillonite, saponite, hectorite, beidellite, stevensite andnontronite; natural micas such as vermiculite and halloysite; syntheticmicas such as swelling mica (swelling mica); and the like. Such layeredsilicates, either synthetic or natural, can be suitably used. The use ofswelling smectite clay minerals and swelling micas are preferred.

The above-listed layered silicates may be used alone or in combination.

In the present invention, the babble growth and gas escaping aresuppressed by the barrier action of the flaky crystals of the layeredsilicate. The use of the layered silicate which includes agglomerates ofhigh-aspect ratio flaky crystals thus results in the formation of a finefoam cell structure with a high expansion ratio. For this reason, theuse of the layered silicate having flaky crystals with an aspect ratioof 100 or above is preferred. Particularly, the use of montmorilloniteincluding flaky crystals with an aspect ratio of about 100 or above, orswelling mica including flaky crystals with an aspect ratio of about 150is more preferred.

Preferably, the interlayer spaces of the layered silicate are renderedhydrophobic. Particularly when a nonpolar resin such as a polyolefinresin is used for the thermoplastic resin, it is preferred tohydrophobicize the interlayer spaces because the layered silicate has ahigh affinity for such a thermoplastic resin.

The following techniques (1)-(3) can be utilized to hydrophobicize theinterlayer spaces.

(1) Ion-exchange of exchangeable cations present in the interlayerspaces of the layered silicate with a cationic surfactant

In general, the exchangeable cations present in the interlayer spaces(i.e., on surfaces of the flaky crystals) are ions such as of sodium orcalcium. These ions are ion-exchangeable with exchanging cations of thecationic surfactant. It accordingly becomes possible to insert variouscationic surfactants having exchanging cations into the interlayerspaces.

Thus, the ion exchange of the exchangeable cations for the exchangingions of a low-polarity cationic surfactant renders crystal surfaces ofthe layered silicate less polar or nonpolar, resulting in the increaseddispersion of the layered silicate in a nonpolar resin.

As stated earlier, in general, exchangeable cations are ions of alkalimetals or alkaline-earth metals, such as sodium or calcium. The usefulexchanging cation is an ion which is more base than or comparable to theexchangeable cation.

In the case where an ion is used which is comparable to the exchangeablecation, the concentration of the exchanging ions may be made higher thanthat of the exchangeable cations.

2) Chemical modification of hydroxyl groups present on the crystalsurfaces of the layered silicate by a compound having at its molecularterminal a functional group having a tendency to couple chemically tothe hydroxyl group or a chemical affinity for the hydroxyl group and/ora reactive functional group

(3) Chemical modification of the crystal surfaces of the layeredsilicate by a compound which contains at least one reactive functionalgroup, other than an anionic site present in its molecular chain, andwhich takes the form of an anionic surface active agent and/or a reagentcapable of anionic surface activation

The preceding techniques (1)-(3) may be utilized alone or in anycombination thereof.

The hydrophobicized layered silicate is suitably used because it showsthe increased dispersion in a nonpolar or low-polarity resin such as apolyolefin resin than before being hydrophobicized.

The cationic surfactant is not particularly specified in type and may beselected from generally-used cationic surfactants. Examples of cationicsurfactants include those comprised chiefly of quaternary ammoniumsalts, quaternary phosphonium salts and the like. The use of quaternaryammonium salts having an alkyl chain with 8 or more carbon atoms ispreferred. Without the inclusion of an alkyl chain of 8 or more carbonatoms, the strong hydrophilicity of alkyl ammonium ions may prevent theinterlayer spaces of the layered silicate from being rendered nonpolaror less polar to a sufficient extent.

Examples of quaternary ammonium salts include lauryl trimethyl ammoniumsalt, stearyl trimethyl ammonium salt, trioctyl ammonium salt, distearyldimethyl ammonium salt, di-cured tallow dimethyl ammonium, distearyldibenzyl ammonium salt and the like.

While not particularly specified, the cation exchange capacity of thelayered silicate is preferably 50-200 milliequivalents/100 g. If it isexcessively low, a smaller amount of a cationic surfactant may beintercalated, via ion exchange, between crystal layers to possiblyresult in the insufficient hydrophobicization of the interlayer spaces.On the other hand, if it is excessively high, the adjacent layers of thelayered silicate may be bound more tightly to each other to result inthe difficulty to delaminate (separate) crystal flakes.

The flaky crystals of the layered silicate serve as barriers to suppressgrowth of bubbles during evolution thereof. Accordingly, the excessivelow loading of the layered silicate may result in the failure to obtaina foam having a fine foam cell structure. The excessive high loadingthereof may result in the reduced bending strength and the increasedproduction cost. In view thereof, the layered silicate is preferablyloaded in the range of 0.1-50 parts by weight, more preferably in therange of 2-10 parts by weight, based on 100 parts by weight of thethermoplastic resin.

To obtain a more uniform thermoplastic foam using a layered silicate,the layered silicate when dispersed in the thermoplastic resinpreferably has an average interlayer spacing (interlayer distance of a(001) plane of the layered silicate when determined by X-raydiffractometry) of over 60 Å.

The type of the thermoplastic resin is not particularly specified. Thepreferred examples of useful thermoplastic resins include polyolefinresins, EVA resins, polystyrene resins, vinyl chloride resins, ABSresins, polyvinyl butyral resins and various rubbers. The use ofcrystalline resins such as polyolefin resins is more preferred.

Due to the presence of crystal sites, the crystalline resin in anunmolten state shows a high shape-retaining effect. The crystallineresin is thus easier to retain a shape of a foam during expansion involume of the below-stated chemical substance in a composite materialcomprising the thermoplastic resin and the layered silicate.

The polyolefin resin for use in the present invention is notparticularly specified in type, examples of which include a homopolymerof ethylene, propylene or α-olefin; a copolymer of ethylene andpropylene, a copolymer of ethylene and α-olefin, a compolymer ofpropylene and α-olefin, a copolymer of two or more α-olefins and thelike. Examples of α-olefins include 1-butene, 1 -pentene, 1 -hexene,4-methyl-1 -pentene, 1 -heptene, 1 -octene and the like.

These polyolefin resins may be used alone or in any combination thereof.

The molecular weight and molecular weigh distribution of the polyolefinresin is not particularly specified. Its weight average molecular weightis preferably in the range of 5,000-5,000,000, more preferably in therange of 20,000-300,000. Its molecular weight distribution (weightaverage molecular weight Mw / number average molecular weight Mn) ispreferably in the range of 2-80, more preferably in the range of 3-40.

Other types of polymer compounds, if appropriate, may be alloyed orblended with the thermoplastic resin. For example, a polymer compoundincorporating a grafted maleic or other carboxylic acid may be added ina small amount to increase in advance an affinity between thethermoplastic resin and the layered silicate.

When necessary, suitable additives may be added to the polyolefinicresin to impart desired physical properties thereto. Examples of suchadditives include an antioxidant, light stabilizer, UV absorber,lubricant, flame retardant antistatic agent and the like. A substanceserviceable as a crystal nucleating agent can also be added in a smallamount to finely divide crystals so that the uniformity of physicalproperties is enhanced.

Where the thermoplastic resin is a crystalline resin, any arbitraryorganic or inorganic gas which exists in a gaseous state within therange from (melting point−20°C.) to (melting point+20°C.) can be used asthe chemical substance for insertion into the interlayer spaces of thelayered silicate used. Where the thermoplastic resin is a noncrystallineresin, any arbitrary organic or inorganic gas which exists in a gaseousstate within the range from (glass transition point−20°C.) to (glasstransition point+20°C.) can be used as the chemical substance forinsertion into the interlayer spaces of the layered silicate used.Examples of suitable gases include carbon dioxide (carbonic acid gas),nitrogen, oxygen, argon or water; organic gases such as flon,low-molecular weight hydrocarbons, chlorinated aliphatic hydrocarbons,alcohols, benzene, toluene, xylene, mesitylene; or the like. Inparticular, a gas is suitably used which assumes a gaseous form atordinary temperature (23° C.) and pressure (atmospheric pressure).

Examples of low-molecular weight hydrocarbons include pentane, butaneand hexane. Examples of chlorinated aliphatic hydrocarbons includemethyl chloride and methylene chloride. Also useful are variousfluorinated aliphatic hydrocarbons.

Carbon dioxide is suitable for use as the chemical substance because iteliminates the need of gas recovery and is safe to handle. Carbondioxide can be changed to a supercritical state at relatively lowtemperature and pressure and, while in the form of a supercriticalfluid, acts in an effective manner to promote dispersion of the layeredsilicate. The supercritical state refers to a state in which temperatureand pressure are above a critical point of the chemical substance to beimpregnated. In the super-critical state, the chemical substance has novapor-liquid transition. The supercritical fluid exhibits propertiesintermediate between the gas and the liquid, such as a high thermalconductivity, low diffusion rate and low viscosity. Accordingly, thesupercritical fluid is suited in dispersing the layered silicate.

The chemical substance may take a liquid form at ordinary temperature.Examples of such chemical substances include saturated hydrocarbons suchas pentane, neopentane, hexane and heptane; chlorine compounds such asmethylene chloride, trichloroethylene and dichloroethane; and fluorinecompounds such as CFC-11, CFC-12, CFC-13 and CFC-141b.

Various techniques can be utilized to impregnate the chemical substanceinto the interlayer spaces of the layered silicate incorporated,together with the thermoplastic resin, in the composite material. Anillustrative technique involves introducing, in the form of a gas, thechemical substance in a closed autoclave for subsequent pressurizing.Using this technique, the pressure and temperature can be readilycontrolled. Another technique involves loading the thermoplastic resinin a melt extruder with a vented screw and supplying the gas from amiddle portion of a cylinder to a vent portion. In this instance, if theresin while in a molten state is pressure sealed, the chemical substancecan be impregnated in a more effective manner into the compositematerial containing the thermoplastic resin and the layered silicate toinsure continuous production of thermoplastic foams.

In the case of using a gas for the chemical substance that assumes agaseous form at ordinary temperature and pressure, when the chemicalsubstance is impregnated into the composite material containing thethermoplastic resin and layered silicate, such a gas is preferablymaintained at a pressure of 9.8×10⁵ Pa or above, more preferably 9.8×10⁶Pa or above.

The conditions required to change the gas that assumes a gaseous form atordinary temperature and pressure, as the chemical substance, to asupercritical fluid vary depending upon the type of the chemicalsubstance used. As described above, carbon dioxide shows a supercriticalproperty under relatively gentle conditions and can be changed to asupercritical fluid at 60° C. and 60 atmospheric pressure, for example.

No particular limitation is given to the temperature at which thechemical substance is impregnated into the composite material containingthe thermoplastic resin and layered silicate, unless it causesdeterioration of the composite material. However, at highertemperatures, a larger amount of the chemical substance dissolves in thecomposite material containing the thermoplastic resin and layeredsilicate to result in obtaining a higher expansion ratio. Accordingly,the higher impregnation temperature is preferred. In order to establisha satisfactory foaming environment, it is more preferred that thetemperature is maintained within the range from (melting point−20° C.)to (melting point+20° C.), if a crystalline resin is used for thethermoplastic resin, or within the range from (glass transitionpoint−20° C.) to (glass transition point+20° C.) if a noncrystallineresin is used for the thermoplastic resin.

If the impregnation temperature of the chemical substance is higher than(melting or glass transition point+20° C.), a molecular motion of thethermoplastic resin becomes active to result in the increased tendencyof the chemical substance once dissolved in the composite material toescape therefrom. On the other hand, if the impregnation temperature ofthe chemical substance is lower than the melting or glass transitionpoint, the molecular motion of the thermoplastic resin becomesinsufficient and in some cases prevents the sufficient dissolution ofthe chemical substance in the composite material.

The thermoplastic foam in accordance with the present invention isproduced by impregnating the aforementioned chemical substance into thecomposite material containing the thermoplastic resin and the layeredsilicate and then allowing the chemical substance to expand in thecomposite material. The technique used to effect expansion in volume ofthe chemical substance is suitably chosen depending upon the type of thechemical substance used, and may comprise impregnating the chemicalsubstance in the form of a gas into the composite material under arelative high pressure, and then reducing the pressure or applying heat.

The temperature at which the chemical substance is caused to expand involume within the composite material is not particularly specified.Preferably, the temperature is maintained within the range from (meltingpoint−50° C.) to (melting point+10° C.) if the thermoplastic resin is acrystalline resin or within the range from (glass transition point−50°C.) to (glass transition point+50° C.) if the thermoplastic resin is anoncrystalline resin. That is, if the temperature of volume expansion ishigher than (melting point+10° C.) or (glass transition point+50° C.),the gas once dissolved is allowed to escape readily, resulting in thedifficulty to sustain a desired cell structure. On the other hand, ifthe temperature of volume expansion is below (melting or glasstransition point−50° C.), a molecular motion of the thermoplastic resinis restricted to result in the failure to obtain a high expansion ratio.

As stated earlier, in a broad aspect of the present invention, a foamstructure is obtained by impregnating a chemical substance that assumesa gaseous form at ordinary temperature and pressure into a thermoplasticresin composition containing 100 parts by weight of a thermoplasticresin and 0.1-50 parts by weight of a layered silicate under a highpressure within an injection molding machine having a cavity, injectingthe thermoplastic resin composition impregnated with the chemicalsubstance into the cavity of the injection molding machine, and thenenlarging the cavity. The useful examples of thermoplastic resins,layered silicates and gases that assume a gaseous form at ordinarytemperature and pressure were previously listed. However, the use ofcarbon dioxide is preferred because it eliminates the need of gasrecovery and is safe to handle.

The above-described technique can also be utilized here to impregnatethe chemical substance under a high pressure within the injectionmolding machine.

The thermoplastic resin composition impregnated with the chemicalsubstance is injected into the mold cavity of the injection moldingmachine, followed by enlargement of the cavity.

Preferably, the cavity is enlarged in a direction perpendicular to aparting plane of an injection mold by simply moving a moveable mold halfbackward. However, the cavity may be enlarged in a direction parallel tothe parting plane as by using a slide core, if the occasion permits.

The size of the cavity after being enlarged is suitably adjusteddepending upon the desired expansion ratio of the end foam. If the sizeis excessively small, the enlargement results in the failure to impartproperties (light weight, heat insulating or other properties) neededfor foams. On the other hand, if the size is excessively large, thecavity after enlargement may be insufficiently filled with thethermoplastic resin composition to result in the failure to impart thedesired expansion ratio and shape of the end foam. Accordingly, thecavity is preferably enlarged to 2-30 times of its size before beingenlarged.

The time required to enlarge the cavity may be varied depending upon thedesired expansion ratio and shape of the resulting foam and upon theparticular tensile viscosity of the thermoplastic resin compositionused. Although the means for enlarging the cavity has its limit, ashorter operation thereof is preferred. This permits expansion of thechemical substance while having a high tensile viscosity and preventscell breakage, so that a foam having a fine cell structure can beobtained. It is accordingly preferred that the enlargement completeswithin 0.5-5 seconds.

Preferably, the chemical substance is brought to a supercritical statewhen its impregnation is carried out within the injection moldingmachine. This further promotes dispersion of the flaky crystals of thelayered silicate.

As stated above, the thermoplastic resin composition impregnated withthe chemical substance is first injected into the cavity. When thecavity is subsequently enlarged, the pressure within the cavity israpidly released. This provides an energy sufficient to overcome anelectrical attraction force between the layered silicates. As a result,the flaky crystals of the layered silicate can be separated from eachother. As also stated above, in the case where the chemical substancewhile in a supercritical fluid state has been impregnated into thethermoplastic resin composition, the subsequent enlargement of thecavity causes the chemical substance to undergo a rapid change to agaseous state. In this instance, the change of the chemical substancefrom the supercritical state to the gaseous state is accompanied byrapid and large volume expansion. This provides an energy sufficient toseparate the flaky crystals of the layered silicate and thus furtherpromotes dispersion of the flaky crystals.

One embodiment of the production method of the present invention inwhich the enlargement of the cavity results in the formation of a cellstructure is now described with reference to FIGS. 4-6.

FIG. 4 is a sectional view, illustrating one example of an injectionmolding machine for use in this embodiment.

In FIG. 4, reference numerals 11, 12 and 16 represent an injectionmolding machine, an injection mold and a vent portion, respectively.

As shown in FIG. 4, the injection molding machine for use in thisembodiment includes an injection machine main body 11 and an injectionmold 12.

The injection machine main body 11 includes a cylinder 14 with abuilt-in screw 13, a hopper 15 through which the thermoplastic resincomposition is fed into the cylinder 14, and a vent portion 16 throughwhich the chemical substance from a gas forcing device 61 is introducedinto the cylinder 14.

FIG. 5 is a sectional view, showing the injection mold, for use in thisembodiment, in the condition of being closed by clamping. FIG. 6 is asectional view, showing an injection mold cavity in the condition ofbeing enlarged.

In FIGS. 5 and 6, reference numerals 12 and 23 denote an injection moldand a mold cavity, respectively.

As shown in FIGS. 5 and 6, the injection mold for use in this embodimentincludes a stationary mold half 21 and a movable mold half 22. Whenclamped, the cavity 23 is defined between the stationary half 21 and themovable half 22.

In this embodiment, the aforementioned thermoplastic resin compositionis fed into the hopper 15 shown in FIG. 4, and the chemical substancethat assumes a gaseous form at ordinary temperature and pressure isintroduced by the gas forcing device 61 into the cylinder 14 through thevent portion 16.

An interior pressure of the cylinder is now increased. Also, thechemical substance is impregnated into the thermoplastic resincomposition at a temperature and pressure sufficient to bring thechemical substance to a supercritical state.

In this case, if the thermoplastic resin composition while in a moltenstate is pressure sealed, the chemical substance while pressurized or ina supercritical state can be impregnated into the thermoplastic resincomposition more effectively.

The thermoplastic resin composition 25 impregnated with the chemicalsubstance is then injected into the cavity 23 through a sprue 24 of theinjection mold 12.

Subsequently, the movable half 22 of the injection mold 12 is shiftedbackward, as shown in FIG. 6, to enlarge the cavity 23.

This causes expansion of the thermoplastic resin composition injectedinto the cavity 23 to result in obtaining a thermoplastic foam.

FIG. 7 FIG. 4 is a sectional view, illustrating another example of aninjection molding machine for use in this embodiment.

In FIG. 7, reference numeral 17 denotes an airtight container. As shownin FIG. 7, an injection machine main body 11 includes a cylinder 14 witha built-in screw 13, a hopper 15 through which the thermoplastic resincomposition is fed into the cylinder 14, and the airtight container 17through which the chemical substance from a gas forcing device 70 isintroduced into the cylinder 14.

In this embodiment, the aforementioned thermoplastic resin compositionis fed into the hopper 15 of the injection machine main body 11 shown inFIG. 7. Then, the chemical substance that takes a gaseous form atordinary temperature and pressure is forced by the gas forcing device 70into the airtight container 17, impregnated into the thermoplastic resincomposition fed into the hopper 15 either under a high pressure or at atemperature and pressure sufficient to bring the chemical substance to asupercritical state, and injected into the cylinder 14.

If the procedures described with reference to FIGS. 4-6 are followed, athermoplastic foam can be obtained.

In the aforestated further broad aspect of the present invention, athermoplastic foam is obtained by preparing a composite material whichcomprises 100 parts by weight of a thermoplastic resin and 0.1-50 partsby weight of a layered silicate containing a heat decomposable blowingagent between its layers, and heating the composite material to atemperature sufficiently high to cause decomposition of the blowingagent. The types of applicable thermoplastic resins and layeredsilicates are described above.

The heat decomposable blowing agent, as described above, refers to asubstance which decomposes when heated to generate a gas. Examples ofsuch substances include azodicarbonamide, benzene sulfonyl hydrazide,dinitrosopentamethylenetetramine, toluene sulfonyl hydrazide,4,4-oxybis(benzene sulfonyl hydrazide) and the like.

The technique used to incorporate the heat decomposable blowing agentinto the interlayer spaces of the layered silicate is not particularlyspecified. The following techniques can be employed, for example.

(1) A blowing agent is converted to a quaternary amine by allowing ahydrochloric acid to attack to a terminal amine of the blowing agent.The ion-exchanging in water of the quaternary amine with metal ionspreviously contained in the interlayer spaces of the layered silicateresults in the incorporation of the blowing agent in the interlayerspaces. This technique is suitably used because most general-purposeblowing agents have a terminal amine.

(2) The heat decomposable blowing agent is solvated, in water, withmetal ions present in the interlayer spaces of the layered silicate.This technique is also suitably used because, in general, mostgeneral-purpose heat decomposable blowing agents include a site, such asnitrogen and a carbon-carbon double bond, which tend to form acoordinate bond with metal ions.

The heat decomposable blowing agent may be incorporated in theinterlayer spaces of the layered silicate at any temperature whichneither causes deterioration of the composite material nor initiatesdecomposition of the blowing agent.

The temperature at which the heat decomposable blowing agent is causedto expand in the thermoplastic resin is not particularly specified.

The thermoplastic foam obtained in accordance with the present inventionhas uniform and fine foam cells as a result of the barrier action of theflaky crystals of the layered silicate. Accordingly, the thermoplasticfoam in accordance with the present invention is applicable for varioususes. Depending upon the purposes contemplated, suitable changes andmodifications may be made to the thermoplastic foam in accordance withthe present invention. For example, its expansion ratio is notnecessarily required to be high when it is applied for uses where highfoam properties are not necessarily required or a reinforcing effect asa result of dispersion of the layered silicate is mainly sought. Thethermoplastic foam in accordance with the present invention may beheated or pressed to break cells to use as a solid body. Also, thethermoplastic foam obtained in accordance with the present invention maybe used as a master batch for provision in the succeeding fabricationprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view given to explain a structure of alayered silicate for use in obtaining the thermoplastic foam of thepresent invention;

FIG. 2 is an enlarged schematic view which shows a partial structure ofthe layered silicate shown in FIG. 1 in which crystal surfaces facetoward each other;

FIG. 3 is a schematic view which shows a model given to explain how thegas diffusion during formation of foam cells is prevented;

FIG. 4 is a sectional view which shows an injection molding machine foruse in one embodiment of the present invention;

FIG. 5 is a sectional view of an injection mold, for use in oneembodiment of the present invention, in the condition of being closed byclamping;

FIG. 6 is a sectional view of an injection mold shown in FIG. 5, in thecondition of being enlarged;

FIG. 7 is a sectional view which shows an injection molding machine foruse in another embodiment of the present invention; and

FIG. 8 is a schematic constitutional view given to explain an extruderfor use in the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is below described in more detail with referenceto the following non-limiting examples and comparative examples.

(RAW MATERIAL)

(a) Layered Silicate

The following minerals were used for the layered silicate.

-   -   (1) Montmorillonite: montmorillonite manufactured by Hojun Kogyo        Co., Ltd. (product name: Bengel-A)    -   (2) Swelling mica: swelling mica manufactured by Corp Chemical        Co., Ltd. (product name: ME-100)        (b) Layered Silicate Containing a Cationic Surfactant

The following commercial products were used for the layered silicatecontaining a cationic surfactant.

-   -   (1) DSDM-modified montmorillonite: DSDM-modified montmorillonite        manufactured by Hojun Kogyo Co., Ltd. (product name: New S-Ben,        organically-modified montmorillonite prepared via total        ion-exchange of sodium ions present in interlayer spaces of        montmorillonite with distearyl dimethyl ammonium chloride)    -   (2) DSDM-modified swelling mica: DSDM-modified swelling mica        manufactured by Corp Chemical Co., Ltd. (product name: MAE,        organically-modified swelling mica prepared via total        ion-exchange of sodium ions present in interlayer spaces with        distearyl dimethyl ammonium chloride)        (c) Chemical Substance for Impregnation into a Composite        Material

The followings were used for the chemical substance.

-   -   (1) Carbon dioxide (carbonic acid gas)    -   (2) Nitrogen    -   (3) Pentane    -   (4) Xylene    -   (5) Water        (d) Heat Decomposable Blowing Agent for Insertion into the        Interlayer Spaces of the Layered Silicate

The followings were used for the heat decomposable blowing agent.

-   -   (1) Azodicarbonamide (product of Eiwa Kasei Co., Ltd.)    -   (2) Benzene sulfonyl hydrazide (product of Eiwa Kasei Co., Ltd.)        (e) Thermoplastic Resin    -   (1) Polypropylene: (product of Nippon Polychem Co., Ltd.,        product name: EA9, density of 0.91, MFR (melt flow rate)=0.5)    -   (2) Polyethylene: (product of Nippon Polychem Co., Ltd., product        name: HB530, density of 0.96, MFR=0.5)    -   (3) Polyethylene: (product of Nippon Polychem Co., Ltd., product        name: UE320, density of 0.92, MFR=0.7)    -   (4) Polyvinyl butyral: (product of Sekisui Chemical Co., Ltd.,        product name: BH-5, glass transition temperature of 65° C.)        (f) Acid-modified Polyolefin Resin

For the increased affinity between the thermoplastic resin and thelayered silicate and also for comparison with conventional cases, thefollowing acid-modified polyolefin resins were used.

-   -   (1) Maleic anhydride modified polypropylene oligomer: (product        of Sanyo Chemical Industries, Ltd., product name: U-mex 1001,        functional group content=0.23 mmol/g)    -   (2) Maleic anhydride modified polyethylene oligomer: (product of        Sanyo Chem. Ind., Ltd., product name: U-mex 2000, functional        group content=0.92 mmol/g)

EXAMPLES 1-14 AND COMPARATIVE EXAMPLES 1-6

The materials used in Examples 1-14 and Comparative Examples 1-6 arespecified in Table 1.

1) Preparation of sample foams

A thermoplastic resin and a layered silicate in the weight ratiospecified in Table 1 were supplied to an interior of a Labo Plastomill,manufactured by Toyo Seiki Ltd., in which they were melt kneaded at atemperature set at 170° C. As specified in Table 1, the layered silicatewas selected from those described above as either excluding orcontaining a cationic surfactant. In Examples 5-10 and 12 andComparative Examples 2 and 3, the above-described acid-modifiedpolyolefin resin was further added in the proportion specified in Table1, based on 100 parts by weight of the thermoplastic resin, for thepurpose of improving an affinity between the thermoplastic resin and thelayered silicate.

The resulting composite composition was preheated in a melt press at170° C. for 5 minutes and then pressed at a pressure of 9.8 MPa for 1minute to provide a 1 mm thick sheet.

A 3 cm square piece was cut out from the sheet and placed in an closedautoclave. An interior temperature of the autoclave was controlled at atemperature 10° C. higher than a melting point or glass transition pointof the thermoplastic resin. Subsequently, a pressurized carbonic acidgas, nitrogen or water vapor was introduced into the autoclave. Theinterior pressure of the autoclave was kept at 1.67 MPa for 30 minutes.Then, the interior temperature of the autoclave was set at a level 10°C. lower than the melting or glass transition point of the thermoplasticresin. In this condition, the gas present in the autoclave was rapidlydischarged therefrom to return the interior pressure of the autoclave toan ordinary pressure. This resulted in obtaining sample foams.

EXAMPLES 15-24 AND COMPARATIVE EXAMPLE 9

(1) Preparation of intercalation compound containing a heat decomposableblowing agent in its interlayer spaces

20 g of Hojun Kogyo montmorillonite or Corp Chemical swelling mica wasincorporated in 1 L distilled water. A stirring motor was operated toeffect stirring of the mixture at ordinary temperature for 1 hour toobtain a dispersed slurry. 40 g of azodicarbonamide or benzene sulfonylhydrazide, as a heat decomposable blowing agent, was added to thedispersed slurry which was subsequently stirred at ordinary temperaturefor 30 minutes by the stirring motor. 8 g of distearyl dimethyl ammoniumchloride was further added to the slurry which was then stirred atordinary temperature for 1 hour by the stirring motor. The resultingslurry was subjected to centrifugal solid-liquid separation. Thesubsequent vacuum drying at 60° C. for 24 hours resulted in obtaining anintercalation compound containing the heat decomposable blowing agentbetween its layers.

The thermoplastic resin and the layered silicate specified in type inTable 1 were supplied in the weight proportion also specified in Table 3to an interior of a Labo Plastomill, manufactured by Toyo Seiki Ltd., inwhich they were melt kneaded at a temperature set at 170° C. In Examples15-24, montmorillonite or swelling mica which contains azodicarbonamidebetween layers was used.

In Examples 19-23 and Comparative Examples 8 and 9, the acid-modifiedpolyolefin resin was further added in the proportion specified in Table3, based on 100 parts by weight of the thermoplastic resin, for thepurpose of improving an affinity between the thermoplastic resin and thelayered silicate.

The resulting composite composition was preheated in a melt press at170° C. for 5 minutes and then pressed at a pressure of 9.8 MPa for 1minute to provide a 1 mm thick sheet. This sheet was immersed in a hotsilicone oil kept at 200° C. for 10 seconds to obtain a foam.

Comparative Examples 7, 8, 10 and 11

The thermoplastic resin and layered silicate containing no heatdecomposable blowing agent between layers were supplied in the weightratio specified in Table 3 to an interior of a Labo Plastomill,manufactured by Toyo Seiki Ltd., in which they were melt kneaded at atemperature set at 200° C. In Comparative Example 8, 5 parts by weightof the acid-modified polyolefin was also added, based on 100 parts byweight of the thermoplastic resin, to increase an affinity between thethermoplastic resin and the layered silicate.

In Comparative Examples 7, 8, 10 and 11, the composite compositioncontaining the ingredients specified in Table 2 and obtained in themanner as described above was pelletized and then supplied, togetherwith the heat decomposable blowing agent specified in Table 2, to aninterior of a Labo Plastomill, manufactured by Toyo Seiki Ltd., in whichthey were subjected to 3-minute melt kneading. The resulting compositematerial was preheated in a melt press at 180° C. for 2 minutes and thenpressed at a pressure of 9.8 MPa for 1 minute to provide a 1 mm thicksheet. This sheet was immersed in a hot silicone oil kept at 200° C. for10 seconds to obtain a foam.

Comparative Example 12

Composition disclosed in Japanese Patent Laying-Open No. Hei 9-183910

Composite material of a solvent-swollen layered silicate and athermoplastic resin

The following compositions were used for the composite composition of asolvent-swollen layered silicate and a thermoplastic resin. 500 g ofDSDM-modified montmorillonite manufactured by Hojun Kogyo Co., Ltd.(product name: New S-Ben D) was added to 5 L xylene (product of WakoPure Chem. Ind., Ltd.). Using a motor-driven stirrer, the mixture wasstirred at ordinary temperature for 2 hours to provide a slurry.Polypropylene (product of Nippon Polychem Co., Ltd., product name: EA9,density of 0.91, MFR=0.5) was extruded at an extrusion temperature of200° C. by a melt extruder. Concurrently, the slurry was introduced froma liquid delivery nozzle disposed along the extrusion line and thexylene was suctioned from a vent port located downstream of the liquiddelivery nozzle along the extrusion line. The composite material wasextruded from a sheet die attached to a front end of the extruder into a1 mm thick sheet serving as a sample for subsequent evaluation.

Comparative Example 13

Composition disclosed in Japanese Patent Laying-Open No. Hei 10-182892

(Composite material of a layered silicate, a thermoplastic resin and anacid-modified oligomer)

A polypropylene resin (product of Nippon Polychem Co., Ltd., productname: EA9, density of 0.91, MFR=1.5), a Hojun Kogyo DSDM-modifiedmontmorillonite (product name: New S-Ben D) and a maleic anhydridemodified polypropylene oligomer (product of Sanyo Chem. Ind., Ltd.,product name: U-mex 1001, functional group content=0.23 mmol /g) in theweight ratio of 80/5/15 were fed into a Toyo Seiki Labo Plastomill inwhich they were melt kneaded at a temperature set at 200° C. Theresulting composite composition was preheated in a melt press at 200° C.for 5 minutes and then pressed at a pressure of 9.8 MPa for 1 minute toprovide a 1 mm thick sheet serving as a sample for evaluation.

Comparative Example 14

Composition disclosed in Japanese Patent Laying-Open No. Hei 8-143697

80 g of Hojun Kogyo montmorillonite (product name: Ben-Gel A), 80 g of5-amino-1H-tetrazole (HAT), 40 g of vinyltriethoxysilane (Vsi), 2 L ofmethyl alcohol and 0.1 L of water were charged into a 3 L round flask.The flask contents were stirred at 60° C. for 24 hours using amotor-driven stirrer, fluxed and passed through a filter paper tocollect a filter cake. This filter cake was vacuum dried at 50° C. for24 hours by a vacuum drier to obtain a composition for use as a layeredsilicate incorporating a blowing agent adsorbed thereto. A sum ofproportions of the blowing agent and silane coupling agent thatrespectively adsorbed to the layered silicate was 45.3 %. Theabove-obtained composition and a polypropylene resin (product of NipponPolychem Co., Ltd., product name: EA9, density of 0.91, MFR=0.5) weremelt kneaded in a Toyo Seiki Labo Plastomill at 160° C. The resultingcomposition was further immersed in a hot silicone oil kept at 180° C.to thereby obtain a sample foam.

Sample Evaluation Method

1) Interlayer spacing of layered silicate

An X-ray diffractometer (product of Rigaku Co., Ltd., product name:RINT-1100) was used to measure a diffraction peak 2θ for layered planesof the layered silicate in the composite material. An interplanarspacing between the flaky crystals of the layered silicate wascalculated from the following Bragg diffraction equation.λ=2d sin θ  (1)

-   -   (where, λ=1.54, d is an interplanar spacing of the layered        silicate, and θ is a diffracted angle.)

The value of d, as derived from the equation (1), was taken as a valuefor an interlayer spacing.

2) Expansion Ratio

An expansion ratio of the foam was calculated from the followingequation (2) wherein a density of the foam was calculated from a buoyantforce exerted thereon by water in which it was submerged.Expansion ratio=density before expansion/density after expansion  (2)

3) Foam cell diameter

A secondary electron reflection type electron microscope (product ofJOEL Ltd., product name: JSM-5800LV) was utilized to observe the foam.50 foam cells were observed for cell diameter and an arithmetic meanthereof was taken as a foam cell diameter of the foam.

(Results)

Evaluation results, i.e., an interplanar spacing of the layeredsilicate, expansion ratio and foam cell diameter, for each of the foamsobtained in Examples and Comparative Examples are shown in Tables 2 and4.

In Examples 1-24, expansion of the chemical substance impregnated in thecomposite material containing the layered silicate resulted in obtainingfoams having a high expansion ratio and a uniform cell diameter. Also,either foam has a foam cell diameter within the range of 10-75 μm, whichvalue is very small for a high expansion-ratio foam.

In contrast, in Comparative Examples 1, 2, 4 and 5, the impregnation ofthe gaseous substance or supercritical fluid into the thermoplasticresin did not lead to high expansion ratios.

For the foams obtained in Examples 1-24, the X-ray diffractometryrevealed no diffraction corresponding to the interlayer spacing. 2θ=1.5,i.e., the interlayer spacing of 60 Å is the detection limit of the X-raydiffractometer used for the measurement. Thus, the X-ray diffractometeris incapable of detecting the interlayer spacing if over 60 Å. From thenature of the measurement, any diffraction must be obtained if theinterlayer spacing is 60 Å. It follows that the layered silicates in thefoams obtained in Examples 1-24 all have the interlayer spacings over 60Å.

As demonstrated by Comparative Example 3, if the amount of the layeredsilicate is excessively large, i.e., if the amount of the layeredsilicate exceeds 50 parts by weight, relative to 100 parts by weight ofthe thermoplastic resin, the interlayer spacing is not caused toenlarge. This is considered due to the barrier action of the layeredsilicate that suppressed diffusion of the gas to an excessive extent.

The utilization of the technique disclosed either in Comparative Example12 (Japanese Patent Laying-Open No. Hei 9-183910) or Comparative Example13 (Japanese Patent Laying-Open No. Hei 10-182892) resulted in thefailure to increase the interlayer spacing to over 60 Å. However,according to Examples 1-24, the interlayer spacing of over 60 Å wasattained without failure.

Comparative Example 14 (Japanese Patent Laying-Open No. Hei 8-143697),while possible to provide a high expansion ratio, provided an averageinterlayer spacing of 28 Å and a very large foam cell diameter of 402μm. This is considered due to the uneven dispersion of the layeredsilicate that led to the insufficient suppression of diffusion of thegas.

TABLE 1 Acid- Blending Layered Thermoplastic modified ProportionsGaseous Silicate A Resin B Polymer C (A/B/C) Substance Ex. 1DSDM-modified EA9 — 1/99/0 Carbon Montmorillonite Dioxide Gas 2DSDM-modified EA9 — 5/95/0 Carbon Montmorillonite Dioxide Gas 3DSDM-modified EA9 — 30/70/0  Carbon Montmorillonite Dioxide Gas 4DSDM-modified EA9 — 5/95/0 Carbon Swelling Mica Dioxide Gas 5Montmorillonite EA9 U-mex 5/90/5 Carbon 1001 Dioxide Gas 6 Swelling MicaEA9 U-mex 5/90/5 Carbon 1001 Dioxide Gas 7 DSDM-modified EA9 U-mex5/90/5 Carbon Montmorillonite 1001 Dioxide Gas 8 DSDM-modified EA9 U-mex5/90/5 Carbon Swelling Mica 1001 Dioxide Gas 9 DSDM-modified HB530 U-mex5/90/5 Carbon Montmorillonite 2000 Dioxide Gas 10  DSDM-modified EA9U-mex 5/90/5 Nitrogen Montmorillonite 1001 Gas 11  DSDM-modified BH-5 —5/95/0 Carbon Montmorillonite Dioxide Gas 12  DSDM-modified HB530 —1/99/0 Carbon Montmorillonite Dioxide Gas 13  DSDM-modified UE320 U-mex5/95/0 Carbon Montmorillonite 2000 Dioxide Gas 14  Mica EA9 U-mex 5/90/5Water 1001 Vapor Comp. Ex. 1 — EA9 — 0/100/0 Carbon Dioxide Gas 2 — EA9U-mex 0/95/5 Carbon 1001 Dioxide Gas 3 DSDM-modified EA9 U-mex 55/35/10Carbon Montmorillonite 1001 Dioxide Gas 4 — HB530 — 0/100/0 CarbonDioxide Gas 5 — BH-5 — 0/100/0 Carbon Dioxide Gas 6 — UE320 — 0/100/0Carbon Dioxide Gas

TABLE 2 Average Interlayer Spacing of Foam Cell Layered ExpansionDiameter Silicate (Å) Ratio (μm) Ex. 1 Over 60 Å 6.5 49 2 Over 60 Å 11.754 3 Over 60 Å 12.3 39 4 Over 60 Å 14.1 59 5 Over 60 Å 16.6 72 6 Over 60Å 20.1 67 7 Over 60 Å 25.4 59 8 Over 60 Å 28.2 65 9 Over 60 Å 15.9 5010  Over 60 Å 14.2 53 11  Over 60 Å 16.3 12 12  Over 60 Å 6.5 49 13 Over 60 Å 16.4 65 14  Over 60 Å 6.5 80 Comp. Ex. 1 — 0.8 Nonuniform 2 —1.9 Nonuniform 3 28 Å 10.6 44 4 — 1.8 Nonuniform 5 — 3.2 Nonuniform 6 —1.9 Nonuniform

TABLE 3 Acid- Blending Layered Thermoplastic modified BlowingProportions Silicate A Resin B Polymer C Agent D (A/B/C/D) Ex. 15Montmorillonite HB530 — 1/99/0/0 Containing Azodicarbonamide Between ItsLayers 16 Montmorillonite EA9 — 5/95/0/0 Containing AzodicarbonamideBetween Its Layers 17 Montmorillonite EA9 — 30/70/0/0 ContainingAzodicarbonamide Between Its Layers 18 Swelling Mica EA9 — 5/95/0/0Containing Azodicarbonamide Between Its Layers 19 Montmorillonite EA9U-mex 5/90/5/0 Containing 1001 Azodicarbonamide Between Its Layers 20Swelling Mica EA9 U-mex 5/90/5/0 Containing 1001 AzodicarbonamideBetween Its Layers 21 Montmorillonite EA9 U-mex 5/90/5/0 Containing 1001Azodicarbonamide Between Its Layers 22 Montmorillonite HB530 U-mex5/90/5/0 Containing 2000 Azodicarbonamide Between Its Layers 23Montmorillonite EA9 U-mex 5/90/5/0 Containing Benzene 1001 SulfonylHydrazide Between Its Layers 24 Montmorillonite BH-5 — 5/95/0/0Containing Azodicarbonamide Between Its Layers Comp. Ex.  7 — EA9 —0/100/0/5  8 — EA9 U-mex 0/95/5/5 1001  9 DSDM-modified EA9 U-mex55/35/10/5 Montmorillonite 1001 10 — HB530 — 0/100/0/5 11 — BH-5 —0/100/0/5 12 13 14

TABLE 4 Average Interlayer Spacing of Foam Cell Layered ExpansionDiameter Silicate (Å) Ratio (μm) Ex. 15 Over 60 Å 5.5 72 16 Over 60 Å10.8 96 17 Over 60 Å 11.4 54 18 Over 60 Å 13.2 89 19 Over 60 Å 15.6 11120 Over 60 Å 19.1 102 21 Over 60 Å 23.2 92 22 Over 60 Å 24.2 80 23 Over60 Å 23.2 80 24 Over 60 Å 19.2 50 Comp. Ex.  7 — 11.2 321  8 — 12.3 481 9 28 Å 20.4 859 10 — 10.5 483 11 — 9.8 200 12 26 Å — — 13 24 Å — — 1428 Å 22.8 402

EXAMPLE 25 AND COMPARATIVE EXAMPLES 15 AND 16

(Raw Material)

The raw material used in the present invention is below described.

Layered Silicate

The following minerals were used for the layered silicate.

Talc (product of Tokuyama Co., Ltd., product name: T68MMR, talc particlediameter of about 5 μm, pellets containing 70% talc)

Montmorillonite (product of Hojun Kogyo Co., Ltd., product name:Bengel-A)

Layered Silicate Containing a Cationic Surfactant

The following material was used for the layered silicate containing acationic surfactant.

DSDM-modified swelling mica (product of Corp Chemical Co., Ltd., productname: MAE-100 =organically-modified swelling mica prepared via totalion-exchange of sodium ions present in interlayer spaces ofmontmorillonite by distearyl dimethyl ammonium chloride)

Thermoplastic Resin Composition

The following compositions were used for the thermoplastic resin.

Random polypropylene (product of Himont Corp., product name: SR256M,density of 0.91, MFR=2.0)

Linear low-density polyethylene (product of Idemitsu Petrochemical Co.,Ltd., product name: Moretech 0238CN, density of 0.916, MFR=2.0)

Polypropylene (product of Nippon Polychem Co., Ltd., product name: EA9,density of 0.91, MFR=0.5)

Acid-modified Polyolefin Resin

For the increased affinity between the theremoplastic resin and thelayered silicate and also for comparison with prior art, the followingcomposition was used.

Maleic anhydride modified polypropylene oligomer (product of SanyoChemical Industries, Ltd., product name: U-mex 1001, functional groupcontent=0.23 mmol/g)

Crosslinking Aid

The following reagent was used to promote crosslinking by ionizingradiation.

Trimethylolpropane triacrylate (product of Aldrich Co.)

Heat Decomposable Blowing Agent

The following reagent was used as the heat decomposable blowing agent.

Azodicarbonamide (product of Otsuka Chemical Co., Ltd., product name:Unifoam AZ-HM)

EXAMPLES AND COMPARATIVE EXAMPLES FOR PREPARATION OF EVALUATION SAMPLESEXAMPLE 25

The random propylene SR256M and linear low-density polyethylene 0238CNin the ratio of 8:2 was mixed in a Toyo Seiki Labo Plastomill for use asa thermoplastic resin. 5 parts by weight of the DSDM-modified swellingmica MAE-100 and 100 parts by weight of the thermoplastic resin weremelt kneaded at a temperature set at 170° C. In order to improve anaffinity between the thermoplastic resin and layered silicate, themaleic anhydride modified polypropylene U-mex 1001 was also added in theamount of 5 parts by weight per 100 parts thermoplastic resin.

Melt kneading was continued while 3 parts by weight oftrimethylolpropane triactylate as a crosslinking agent and 12 parts byweight of azodicarbonamide Unifoam AZ-HM, each based on 100 parts byweight of the thermoplastic resin, were further added.

The resulting composite composition was pressed using a hand press at180° C. for 3 minutes into a 1 mm thick sheet. This sheet was thenexposed to 10 Mrads of an electron beam accelerated at a voltage of 750kV to effect crosslinking. The irradiated sheet was subsequently allowedto expand in a gear oven at 260° C. to obtain an evaluation sample.

Comparative Example 15

The procedure of Example 1 was followed, except that DSDM-modifiedswelling mica and maleic anhydride modified polypropylene were unloaded,to obtain an evaluation sample.

Comparative Example 16

The procedure of Example 1 was followed, except that talc, generallyused as an inorganic filler, was loaded in the amount of 5 parts byweight per 100 parts by weight of the thermoplastic resin, instead ofloading DSDM-modified swelling mica, and maleic anhydride modifiedpolypropylene was unloaded, to obtain an evaluation sample.

(Sample Evaluation Method)

The above-obtained evaluation samples were evaluated in the same manneras in Example 1. The results are given in the following Table 5.

TABLE 5 Comp. Comp. Comp. Ex. 25 Ex. 15 Ex. 16 Ex. 13 Form Foam FoamFoam Sheet Inorganic DSDM-modified — Talc DSDM-modified FillerMontmorillonite Montmorillonite Dosage 10 10 10 — (Mrad) Interlayer >60— — 24 Spacing (Å) Expansion 27.8 22.9 18.1 — Ratio Foam Cell 120 250950 — Diameter (μm)

EXAMPLE 26

The same raw material as in Example 4 was used, except that the chemicalsubstance was changed from carbon dioxide to pentane assuming a liquidform at ordinary temperature, to produce a foam. In the production ofthe foam, pentane was introduced into an autoclave whose interiorpressure was subsequently increased to and maintained at 5.88 MPa for 30minutes. Then, an interior temperature of the autoclave was set at alevel 10° C. lower than the melting point of the thermoplastic resin(EA9) used. In this condition, the gas present in the autoclave wasrapidly discharged therefrom to return the interior pressure of theautoclave to an ordinary pressure. The resulting foam was evaluated inthe same manner as in Example 1. The average interlayer spacing of thelayered silicate, expansion ratio and foam cell diameter were found tobe over 60 Å, 13.2 and 95 μm, respectively.

EXAMPLE 27

The same raw material as in Example 5 was extruded using a processingmachine shown in FIG. 8 to produce a foam. That is, the thermoplasticresin composition was supplied to a single-screw extruder 71 (includinga screw 72 with a diameter of 40 mm and a length/diameter ratio of 30)through a pressure-resistant hopper 76 in the processing machine shownin FIG. 8. A pressure pump 73 was used to force, in the form of carbondioxide under a pressure of 7.84 MPa, the chemical substance into a gasinlet port 75 located along a liquid transport portion 74. The carbondioxide under such a pressure was allowed to dissove in thetheremoplastic resin composition in an amount of about 9% by weight.

In this instance, a high-pressure sealing mechanism of a screw shaft, apressure-resistant structure of the hopper and the molten thermoplasticresin composition residing near the extruder served to maintain thecarbon dioxide inside the extruder in a dwell condition. Thethermoplastic resin composition fed to the extruder 71 was then meltkneaded therein sufficiently under the following conditions; an outputrate of 2 kg/hour, a screw revolution speed of 10 rpm and a cylindertemperature of 200° C. Subsequently, the thermoplastic resin compositionwas passed through a front end of a die 77 kept at a temperature ofabout 120° C. The resin was extruded from the die 77 into a rod shape toproduce a foam. This foam was evaluated in the same manner as inExample 1. As a result, the interlayer spacing of the layered silicate,expansion ratio and foam cell diameter were found to be over 60 Å, 13.2and 95 μm, respectively.

EXAMPLES 28-35

Polypropylene (product of Nippon Polychem Co., Ltd., product underdesignation of “EA 9”, density of 0.91 g/cm³, MFR=0.5 g/10 minutes),polyvinyl butyral (product of Sekisui Chemical Co., Ltd., product underdesignation of “BH-5”, glass transition temperature of 65° C.),montmorillonite prepared via total ion-exchange of sodium ions presentbetween layers by distearyl dimethyl ammonium chloride (product of HojunKogyo Co., Ltd., product name “New S-Ben D”, denoted in Table 6 asDSDM-modified montmorillonite), swelling mica prepared via totalion-exchange of sodium ions present between layers by distearyl dimethylammonium chloride (product of Corp Chemical Co., Ltd., product underdesignation of “MAE”, denoted in Table 6 as DSDM-modified mica),montmorillonite (product of Hojun Kogyo Co., Ltd., product name “BengelA”), swelling mica (product of Corp Chemical Co., Ltd., product underdesignation of “ME-100”), maleic anhydride modified polypropyleneoligomer (product of Sanyo Chemical Industries, Ltd., product name“U-mex 1001”, functional group content=0.23 mmol/g), respectively in theamounts specified in Table 6, were fed to the hopper 15 of the injectionmolding machine shown in FIG. 7.

Meanwhile, a carbon dioxide (CO₂) or nitrogen (N₂) gas was deliveredfrom the gas forcing device 70 to the closed container 17, andimpregnated under an atmosphere of 10 MPa and 60° C. (in the case ofCO₂, in a supercritical condition and in the case of N₂, in a highlypressurized condition) into the thermoplastic resin composition fed toan interior of the hopper 15. The gas-impregnated composition was thenintroduced into the cylinder 14 controlled at a temperature of 250° C.,in which it was melt kneaded and metered by the screw at a revolutionspeed of 50 rpm. The melt was then injected at a rate of 100 mm/sec intoa disc-shaped cavity having a diameter of 250 mm and a width of 3 mm anddwelled for 20 seconds. Thereafter, the cavity 23 was enlarged in thefashion as shown in FIG. 6, to a width of 45 mm at 1 second and thencooled for 30 seconds to obtain a foam.

The foams obtained in Examples 28-35 were evaluated in the same manneras in Example 1 and also evaluated for thermal conductivity according tothe procedure described below. The results are given in the followingTable 6. In Table 6, the evaluation results of the preceding ComparativeExample 14 are also given for a comparative purpose.

(Thermal Conductivity)

The obtained foams were evaluated for thermal conductivity by a thermalconductivity meter (product of Eiko Seiki Co., Ltd., model type“HC-072”) with a hot plate set at 50° C. and a cool plate set at 20° C.

TABLE 6 Example Comp. 28 29 30 31 32 33 34 35 Ex. 14 CompositionThermoplastic Polypropylene 100 100 95 95 95 95 95 — Followed (Parts byResin Polyvinyl — — — — — — — 100 Japanese Weight) Butyral PatentLayered DSDM-modified 43 — — — 5 — 5 — Laying- Silicate MontmorilloniteOpen No. DSDM-modified — 5 — — — 5 — 5 Hei 8- Swelling Mica 143697Montmorillonite — — 5 — — — — — Swelling Mica — — — 5 — — — —Acid-modified Polypropylene — — 5 5 5 5 5 — Oligomer Chemical SubstanceCO₂ CO₂ CO₂ CO₂ CO₂ CO₂ N₂ CO₂ Evaluation Average InterlayerSpacing >60 >60 >60 >60 >60 >60 >60 >60 28 Results (Å) Foam CellDiameter (μm) 39 59 72 67 59 65 53 12 300 Thermal Conductivity 0.0710.065 0.065 0.055 0.054 0.048 0.057 0.061 0.098 [W/(m · k)]

As a result of evaluation, all of the foams obtained in Examples 28-35were found to exhibit average interlayer spacings of over 60 Å, thedetection limit of the X-ray diffractometer used, have uniform and finefoam cells with cell diameters in the range of 12-72 μm, and show goodheat insulation performances, i.e., thermal conductivities in the rangeof 0.054-0.071 W/(m·K). In contrast, the foam obtained in ComparativeExample 14 was found to exhibit a narrow average interlayer spacing of28 Å, have a very large foam cell diameter of 300 μm, and show a highthermal conductivity of 0.098 W/(m·K).

Effects of the Invention

As stated above, the thermoplastic foam in accordance with the presentinvention contains 100 parts by weight of a thermoplastic resin and0.1-50 parts by weight of a layered silicate which has an averageinterlayer spacing of over 60° C. when determined by an X-raydiffractometry. This constitution assures enhanced dispersion of flakycrystals of the layered silicate in the foam. Thus, the propertiesexpected for the foam by addition of a layered silicate, e.g., heatresistance, flame retardance and dimensional stability, can be furtherenhanced by the even dispersion of the flaky crystals of the layeredsilicate.

Also, in the foam production, such flaky crystals of the layeredsilicate act as gas barriers during formation of a foam cell structure.This results in the provision of a foam which has uniform and fine foamcells evenly dispersed therein and thus has elasticity or otherproperties uniform throughout the foam.

The properties of the foam, such as heat insulation performance,compressive strength and bending creep, may drop if X/(Y−1)^(1/3)exceeds 30 μm, where X is an average cell diameter (μm) and Y is anexpansion ratio.

In the present invention, a polyolefin resin can be used for theaforementioned thermoplastic resin. In such a case, a polyolefin foamcan be provided having improved properties as a result of evendispersion of the layered silicate.

At least one selected from swelling smectite clay minerals and swellingmicas can be suitbaly used for the layered silicate. If this is thecase, a cell diameter can be further reduced by the increased dispersionof such a mineral and its action to serve as a nucleating agent duringcell formation. The increased mechanical strength also results.

In the production method of a thermoplastic foam in accordance with thepresent invention, a volume-expansible chemical substance is impregnatedinto interlayer spaces of a layered silicate in a composite materialcontaining 100 parts by weight of a thermoplastic resin and 0.1-50 partsby weight of the layered silicate and the chemical substance is allowedto expand in volume in the composite material so that a cellularstructure is formed. In this instance, the flaky crystals of the layeredsilicate serves as barriers against expansion in volume of the chemicalsubstance. This prevents excessive release or localized nonuniformexpansion of the gaseous chemical substance and allows even dispersionof the flaky crystals of the layered silicate and thus even distributionof foam cells. Accordingly, a thermoplastic foam excellent inproperties, such as strength and heat resistance, can be readilyprovided. Also, because the method does not use a solvent, a troublesomeprocess for removing a residual solvent is not required.

In the case where a composite material is prepared containing 100 partsby weight of a thermoplastic resin and 0.1-50 parts by weight of alayered silicate incoporating a heat decomposable blowing agent betweenits layers and the composite material is heated to a temperaturesufficiently high to cause decomposition of the blowing agent so that acellular structure is formed, the flaky crystals of the layered silicateact as barriers against a gas produced via thermal decomposition of theblowing agent. In the similar manner, this allows even dispersion of theflaky crystals of the layered silicate and thus even distribution offine foam cells. Accordingly, a thermoplastic foam can be provided whichhas improved mechanical strength or other properties uniform throughoutthe foam as a result of even dispersion of the layered silicate therein.

Also in the method wherein a chemical substance that assumes a gaseousform at ordinary temperature and pressure is impregnated under a highpressure into a thermoplastic resin composition containing 100 parts byweight of a thermoplastic resin and 0.1-50 parts by weight of a layeredsilicate within an injection molding machine having a cavity and,subsequent to injection of the thermoplastic resin composition into thecavity, the cavity is enlarged, the flaky crystals of the layeredsilicate act as barriers resulting in even dispersion of the flakycrystals of the layered silicate and thus even distribution of fine foamcells. Accordingly, a thermoplastic foam can be provided which hasimproved mechanical strength or other properties uniform throughout thefoam as a result of even dispersion of the layered silicate therein.Dispersion of the layered silicate can be achieved more effectivelyparticular when the chemical substance while in its suprecritical stateis impregnated into the composition.

Also in the present invention, an increase in heat deformationtemperature due to restriction of molecular chains is expected toresult. The effect of suppressing diffusion of a combustion gas and thenucleating effect of the inorganic crystals are also expected.Accordingly, the heat resistance, flame retardance, dimensionalstability or other properties of a thermoplastic foam can be markedlyimproved.

1. A method for production of a thermoplastic foam comprising the stepsof: impregnating a volume-expansible chemical substance into interlayerspaces of a layered silicate in a composite material containing 100parts by weight of a thermoplastic resin and 0.1-50 parts by weight ofthe layered silicate; and allowing said chemical substance to expand involume within the composite material for formation of cells so that athermoplastic foam is obtained, wherein said layered silicate has anaverage interlayer spacing of over 60 Å when determined by X-raydiffractometry.
 2. The method for production of a thermoplastic foam asrecited in claim 1, characterized in that said step of impregnating thechemical substance is carried out by impregnating under a high pressurethe chemical substance that assumes a gaseous form at ordinarytemperature and pressure, and the chemical substance is allowed toexpand in volume within the composite material by vaporizing thechemical substance within the composite material.
 3. The method forproduction of a thermoplastic foam as recited in claim 2, characterizedin that said chemical substance that assumes a gaseous form at ordinarytemperature and pressure is in its supercritical condition impregnatedinto the composite material.
 4. A method for production of athermoplastic foam comprising the steps of: preparing a compositematerial comprising 100 parts by weight of a thermoplastic resin and0.1-50 parts by weight of a layered silicate containing a heatdecomposable blowing agent between its layers; and heating saidcomposite material to a temperature sufficiently high to causedecomposition of said blowing agent to form a cellular structure, sothat a thermoplastic foam is obtained in which said layered silicate hasan average interlayer spacing of over 60 Å when determined by an X-raydiffractometry.
 5. A method for production of a thermoplastic foamcomprising the steps of: impregnating a volume-expansible chemicalsubstance into a thermoplastic resin composition containing 100 parts byweight of a thermoplastic resin and 0.1-50 parts by weight of a layeredsilicate under a high pressure within an injection molding machinehaving a cavity; and subsequent to injection of the thermoplastic resincomposition into the cavity, allowing the cavity to enlarge, so that athermoplastic foam is obtained in which said layered silicate has anaverage interlayer spacing of over 60 Å when determined by an X-raydiffractometry.
 6. The method for production of a thermoplastic foam asrecited in claim 3, characterized in that said chemical substance is inits supercritical state impregnated into the thermoplastic resincomposition within the injection molding machine.
 7. The method forproduction of a thermoplastic foam as recited in claim 5, whereininterlayer spaces of said layered silicates are hydrophobicized.